Scanning projection mask aligners of the type described in U.S. Pat. No. 4,068,947 entitled "Optical Projection and Scanning Apparatus" utilized in microlithography incorporate an unique ring field optical system which forms a well corrected image of an object within an off-axis annular field. Such an optical system is described in U.S. Pat. No. 3,748,015 entitled "Unit Power Imaging Catoptric Anastigmat". Effective use of such systems require an illumination system which efficiently illuminates the annular field. One such illumination system which is described in U.S. Pat. No. 4,241,390 entitled "System For Illuminating An Annular Field" utilizes an arcuate shaped source of illumination i.e. an arcuate capillary mercury lamp.
A limiting factor in the use of arcuate capillary lamps is the amount of light available from such lamps relative to a short arc lamp. However, short arc lamps do not inherently have the arcuate shape and uniformity necessary for use in ring field systems. Most techniques for converting light output from a short arc lamp to a desired shape suffer from inefficient use of light, image nonuniformity, complexity and difficulty of fabrication. The present invention overcomes the foregoing mentioned problems.